EXPANSION BUS ARCHITECTURE

Electronic signals need a medium to travel on from one location to another. The motherboard is really a circuit board composed of little electronic data paths that allow all those zeroes and ones to travel from one location to another. These little ‘highways’ are known as the motherboard’s bus or I/O bus. The motherboard’s I/O bus leads to expansion buses. The expansion buses are narrow slots on the motherboard, and these buses have different architectures that accept integrated circuit boards, otherwise known as cards. These circuit boards, or cards, are used to communicate with devices, such as monitors, printers, modems, and CD-ROM writers.

There are different buses for memory, processors, addresses, and expansion slots. Each of these buses may have different bus widths or number data paths associated with their architecture. In simple terms, if you are looking at an older motherboard that will only accept 8 bits of information at a time and a 16-bit processor to connect to that motherboard, a bottleneck will occur. The motherboard cannot utilize the full 16 bits of information from the processor.

Table 15.5 lists the most popular expansion slots. An important note for the test: most modern motherboards utilize Accelerated Graphics Port (AGP), Peripheral Component Interconnect (PCI), and Industry-Standard Architecture (ISA) slots.

Table 15.5: Popular Expansion Slots

Bus/Slot	Bits	Comments
ISA	8 or 16 bits	Operates at 8MHz or 8.33MHz.
EISA	32 bits	Supports PnP and bus mastering. ISA slot compatible.
VL-Bus	32 bits	Supports bus mastering. Compatible with ISA.
MCA	16 or 32 bits	Supports PnP and bus mastering. Older, proprietary architecture.
PCI	32 bits	Supports PnP, bus mastering, and burst mode. Utilizes a host bridge to communicate with other expansion slots.
PCI-2	64 bits	Supports PnP and bus mastering. PCI slot compatible.
AGP	32 or 64 bits	Designed for accelerated graphics and video processing.

PCI 32 bit/64 bit

Currently, PCI is by far the most commonly used bus standard for NICs, sound cards, and modems. An expansion bus technology created by Intel, the PCI bus is designed to sync up with the clock speed of the system’s CPU.

There are two main PCI bus implementations: PCI 32-bit bus and PCI 64-bit bus. Most of the motherboards on the market today implement a PCI 32-bit bus that runs at 33MHz. The PCI 32-bit bus is able to access up to 4GB of memory.

A PCI 64-bit bus runs at clock speeds of 33MHz and 66MHz, and has a throughput rate of up to 133Mbps. It uses double 32-bit PCI cycles, called Dual Address Cycles (DAC), which allows a 64-bit PCI bus to access up to 17 billion gigabytes of memory space. What does this mean in simple terms? PCI 64-bit running at 66MHz speed offers greater bandwidth and throughput, which in turn provides better performance for such technologies as Ethernet or graphics. PCI technology is designed to transmit data at 32 bits at a time with a 124-pin connection, and 64 bits at a time using a 188-pin connection.

Accelerated Graphics Port (AGP)

The Accelerated Graphics Port (AGP) specification was designed to offer faster and clearer display of graphical images, such as 3D images and video. Developed by Intel, AGP is based on PCI technology. However, unlike PCI, AGP works on its own point-to-point dedicated channel and allows a graphics controller to directly access a computer system’s RAM in order to provide the faster production of images to the monitor. AGP technology uses memory dynamically. When the memory is not being used by AGP for such things as rendering, texturing, alpha blending, z-buffering, or the general production of images, it is restored to the Operating System (OS) for other purposes.

AGP 2x, 4x, and 8x

AGP technology comes in several specifications. To the end consumer or user, these specifications are better known as acceleration speeds. In simple terms, one can purchase an AGP video card at various rates of acceleration. The higher the acceleration rate, the better the card.

There are two main Intel specifications for AGP that you should be aware of. They are:

AGP Specification Revision 2.0: This specification defines interfaces supporting AGP 1x and 2x.

AGP Specification 3.0: This specification defines AGP 4x and 8x technology. With AGP 8x, it is possible to deliver over 2.1GB. This specification was developed to handle today’s bandwidth-hungry applications and those of the near future.

Universal Serial Bus (USB)

Universal Serial Bus (USB) is a fairly new plug-and-play architecture that uses the PCI bus to communicate between the CPU and memory, and utilizes a 12 million bits per second (Mbps) data transfer rate. USB allows one to attach many low-speed devices to a computer without the need for an expansion card. Devices such as mice, keyboards, printers, and CD-ROMs have been designed with their own built-in controllers that accept the USB standard. You can connect up to 127 peripheral devices to a system with the use of one USB port. In other words, let’s say you want to connect a USB keyboard and a USB mouse to your system. These two USB devices together will only require one system resource Interrupt Request (IRQ).

For the exam, make sure you are able to identify USB ports if presented with a picture of a motherboard or system unit. If you require more information regarding USB, the following HowStuffWorks Web site does a great job explaining how USB ports work: http://computer.howstuffworks.com/usb3.htm.

Riser Cards

As computer systems have evolved and the need for more internal system unit space has increased, PCI riser cards have become a welcome solution in the battle for motherboard extension and overall space savings inside a computer system. When a riser card is plugged into a motherboard, it forms a ‘right angle’ with the motherboard as opposed to lying flat above the motherboard. This allows for more technology to be plugged into the motherboard and provides more space overall. PCI riser cards also provide additional slots for both 64-bit or 32-bit adapter cards.

Riser cards allow for faster production of new technologies. For example, in the past, developers and manufactures of technologies had to go through lengthy certification processes to get their technologies certified for ‘on-the-motherboard integration.’ Riser cards allow for technologies to be developed faster by placing them above the motherboard on a riser card. A perfect example of this will be seen shortly in the discussion of the Audio Modem Riser (AMR) slots.

There are many types of riser cards on the market today. Some of the most popular include riser cards for memory modules such as Dual Inline Memory Module (DIMM), RIMM, and Small-Outline Dual Inline Memory Module (SODIMM). (Memory modules will be discussed in Chapter 17.) There are other popular riser cards, such as the Slot 1 riser cards used for Pentium II processors and Slot 2 riser cards used for XEON processors. (Processors will be discussed in more detail in Chapter 16.) There are also riser cards for other technologies, such as audio, modem, Local Area Network (LAN), and USB. If you look closely at the CompTIA A+ 2003 Objectives, you will notice that AMR and Communication Network Riser (CNR) are targeted. For that very reason, we will discuss them next.

Audio Modem Riser (AMR)

Intel created the Audio Modem Riser (AMR) specification. This specification for motherboard architecture allows analog I/O functions to be separate from the motherboard by placing them on a riser card that contains a codec (Compressor/Decompressor) chip. In simple terms, separating analog I/O functions from the motherboard allows designers to develop newer and better technology faster without having to go through the grueling time-consuming certification process for motherboard manufacturer approval and integration. Another extremely important benefit to this riser card technology is the high quality of audio that can be produced as a result of this process.

Communication Network Riser (CNR)

The Communication and Network Riser (CNR) is an Intel-created standard that applies to riser cards. A CNR card is an ATX-compatible PCI riser card that offers logic support for such technologies as audio, modem, LAN, and USB. The whole idea here is to enable developers to better integrate and compact these technologies into a smaller, more scaleable hardware device that makes better use of motherboard resources and system unit space.

The following Adex Electronics, Inc. Web site provides a superb display and explanation of various riser cards: http://www.adexelec.com/riser.htm.

And the following reference is available on the Web in Portable Document Format (PDF). It explains many technical details associated with the ATX form factor riser card specification. Pay special attention to the PCI slot assignments on ATX form factor boards with ATX riser support. http://www.formfactors.org/developer/specs/atx/ATX_Spec_V1_0.pdf.

Note

Both AMR and CNR riser technologies have been identified as targets for the CompTIA A+ 2003 Objectives exam. Make sure you know what they are.

North and South Bridges

PCI architecture is based on the concept of bridging. A PCI bus has a north bridge and a south bridge. The north bridge communicates with the CPU and is used to send signals to devices that run at higher speeds, such as memory and high-speed graphics ports. The south bridge communicates with a super I/O chip and is used to send signals to slower devices, such as ISA slots, COM ports, and LPT ports.